1. Field of the Invention
This invention relates to LED printers utilizing a low resolution print engine for forming high density images and, more particularly, to an image forming method and an apparatus therefor in a LED printer for forming a virtual 2N dpi high density image on a photosensitive medium by effectively turn-on controlling an array of LED elements arranged at a pitch of N dpi on a line in the main scanning direction.
2. Description of the Prior Art
A LED printer has been well known in the art, which has a LED head having an array of LED elements arranged in a row in the main scanning direction. The LED head faces a photosensitive drum and extends along a bus bar thereof. The LED elements are turn-on controlled for one line at a time or in units of blocks according to video data to form a dot matrix image pattern on the photosensitive drum which is moved relatively in the auxiliary scanning direction.
As this type of LED printer, 300 dpi printers which have low density dot pitch have been used extensively because their price is low and also they are compact in construction. In recent years, however, high density dot pitch printers, for instance 600 dpi printers, have been proposed for higher image quality and resolution.
For constructing a 600 dpi printer, however, the lens structure scale and polygon mirror rotation speed are increased to provide a reduced beam diameter, thus inevitably leading to apparatus size increase. Besides, fine and stringent photosensitive drum rotation speed control is necessary, thus leading to complications of the apparatus construction.
Accordingly, there have been attempts to form virtual high density (i.e., 600 dpi) images by making direct use of existing low density engines (for 300 dpi).
As an example, according to the U.S. Pat. No. 5,134,495, the beam light intensity is set such that, as shown in FIG. 10, the potential level L of the beam dots Dn focused on the photosensitive drum is lower than the effective potential level Ls of effective dots G of latent image, and the potential level of the overlapped portions Dg of the beam dots Dn is higher than the level Ls. A technique has been proposed, which is predicated in the fact that by suitably overlapping the beam dots Dn, an effective dot G is not formed in a central area Dc of dot Dn free from overlap but is formed in the overlapped portions Dg. In this technique, when dots are printed in a row with a 300 dpi engine, effective dots G are formed only in the overlapped portions Dg. An effective dot G is formed on each side of a beam dot Dn. In other words, effective dots G are formed between adjacent actual lines at the pitch of 300 dpi, and 600 dpi image can be formed by effective dots G on 600 dpi lines between adjacent 300 dpi lines.
However, since this prior art technique is based on the overlapping of beam dots, it is necessary to use a somewhat large beam diameter, i.e., a beam diameter greater than that in case of N dpi, for the exposure of the photosensitive drum bus bar. Therefore, combining the effective dots G with usual dots leads to difficult control.
Accordingly, in the above prior art technique the laser beam pulse width is varied to use laser beam pulses having a plurality of different beam intensities for control. However, in the LED printer in which the LED elements are turned on for one scanning line at a time or in units of n bits, unlike the laser beam case it is impossible to obtain fine and dense control in units of a bit.
Further, to provide 600 dpi pitch overlap positions the data for producing pulses have to be 600 dpi data, and this, unlike the conversion of 300 dpi data into 600 dpi data, does not provide for any resolution increase.
Furthermore, in the above prior art technique, of the above 600 dpi image data the data of the center bit and eight adjacent bits thereto, i.e., 3.times.3 bit data, are successively taken out with a matching template for comparison with said templates with a large number of bits, thus controlling the beam pulse for producing the center bit in said plate. However, comparing matrix data of 3.times.3 bits, i.e., a total of 9 bits, with templates means that a large number of templates are required for conversion.
Besides, since the comparison is a two-dimensional process in both the main and auxiliary scanning directions, a shift register is necessary for accommodating bit data of three successive main scanning lines adjacent to the center bit. At any rate, the circuit construction is inevitably increased, and also delay of the operation of comparison is inevitable. Therefore, it is impossible to cope with operation speed increase.